Curable silicone compositions have been known since the 1940's and been used in a variety of end use applications. The silicone polymers are silanol end-blocked and are mixed with alkoxy silanes for crosslinking and cure activities. Typically, these materials have condensation cures, that is, they curs through condensation of the silanol end groups on the silicone polymer with water as the by-product. they are typically enclosed in air tight containers along with condensation catalysts to prevent the premature curing, because once removed from the container, for example, a tube, they begin to cure. Also, in some end uses, the materials are used in coating and dipping operations where the coatings are used in open vessels and they therefore tend to increase in viscosity, i.e. premature cure.
Thus, for any end use application that requires preparation time using the silicone compositions, it would be valuable to provide a delay in the cure of the silicone compositions and thus, it is desirable to use a cure that can be taken on command (command cure). In the field of coatings for substrates such as glass or other applications, there has been an increasing need to have a command cure.
Some existing condensation catalysts, like tin and titanate compounds, cure so well at room temperature that they have very short use times. These catalysts are mixed as a system wherein the catalyst and crosslinker are mixed by an operator immediately prior to use. The mixture is then applied shortly after mixing, resulting in time-related restrictions on the manufacturing processes. Such fast cure systems typically have a very short open pot life.
Many room temperature vulcanizing (RTV) silicone products, such as sealants, utilize condensation curing mechanisms to facilitate rapid polymer cross-linking. These mechanisms can be generalized as reactions between a silanol terminated polymer (typically polydimethylsiloxane) and a multi-functional cross-linking agent such as methyltrimethoxysilane with the use of a catalyst:SiOH+(CH3O)3SiCH3→SiOSiCH3(OCH3)3+HOCH3 
Further cross-linking can be obtained by hydrolyzing the remaining functional groups with ambient moisture, which can further react with any remaining cross-linking agent:SiOCH3+H2O→SiOH+HOCH3 
These reactions are often catalyzed by metal complexes, most commonly tin and titanate complexes such as dibutyltindilaurate and tetrabutyltitanate. These reactions are very quick, usually limited only by the amount of ambient moisture available for hydrolysis and as such there is a desire to inhibit the cure rate to give more control over a product. Conventionai tin catalysts also never deactivate, and remain in the product to catalyze the reversal of the aforementioned condensation reactions:SiOSi+H2O→SiOH+HOSi (with tin catalyzing)
These conventional catalysts are less stable and can be easily compromised in the presence of heat, fire or weather. Such a reaction tends to occur in the presence of excess moisture and heat. This is due to the non-active ligands present on the metal catalyst, for example the butyl groups on dibutyltindilaurate. Since these alkyl groups never react, they keep the tin complexes soluble in the siloxane which can lead to tin-catalyzed polymer cleavage.
Some materials, like thin film silicone coatings, achieve a longer pot life using inhibited platinum catalysts, encapsulated platinum catalysts, or very slow cure forms of the precious metal catalysts to achieve the command cure in silicone, known as “addition cure” chemistry. Addition cure refers to the hydrosilyation reaction between a hydride functional silicone and an unsaturated moiety. The inhibited cures with these types of ingredients are much more expensive than the silanol ended polymers used in condensation curing systems, thus making them more expensive products.
It is desirable to have a system where the materials can be open to the air, applied to the surface of a substrate and then the cure mechanism is triggered, while using low-cost products. In many situations, a command cure, which allows for a longer pot life, that could be achieved at relatively low cost, is desired.
The present invention demonstrates that inhibited tin or titanate catalysts can be used in condensation cures and can be command cure systems with long open pot life, yet have relatively fast cure when the cure mechanism is triggered. This combines the advantages of the inhibited addition cure systems (command cure) with the advantages of the condensation cure systems (lower cost). In addition, the inhibited catalyst is completely deactivated which provides for a final product that is more heat, fire, and weather stable, and is not biologically active. Another benefit of a command cure in a condensation system is that adhesion is typically easier to achieve in condensation systems and thus a command curs with lower cost also has the potential to achieve better adhesion.
By inhibiting the reactivity of conventional condensation cure catalysts such as tin or titanate compounds, the catalysts may remain in fully formulated materials without showing catalytic activity, thereby contributing to a longer pot life. Such catalysts may be inhibited using alcohols, mercaptans, and/or chelates.
This technology may be applied to a variety of products including silicone, urethanes (coatings, sealants, plastics), and polyesters (used in urethanes).
The present invention may also fee used for coatings for glass and other applications, such as sealants. Embodiments of the present invention may be used with roof tiles, siding, sealants (construction, marine, home), adhesives, concrete coatings, glass coatings, auto air bags, gaskets, hose & tubing, injection molding, pressure-sensitive release coatings, RTV silicones, and fabric coatings.
Command catalysts of inhibited tin and titanate can be made by any of the techniques known to chemists in the industry. One such technique is a ligand exchange. An example is Bu2SnCl2+2RSH----->Bu2Sn(SR)2+HCl. Another is Bu2Sn (OAc)2+2RSH------->Bu2Sn(SR)2+2 HOAc. Yet another is Sn(OAC)4+4RSH----->Sn(SR)4+4 HOAc. Another example is Ti(OPr)4+HN(CH2CH2OH)3-----> Ti (NCH2CH2O)+4 HOPr. In each case the leaving groups, if it is more volatile, can be stripped out, or if acidic, can be captured by an acceptor to make a salt that can be filtered out. These are but a few examples of the techniques common to the industry used, to make the catalysts used in these formulations.